Recombinant SARS-CoV nsp12 and the use of thereof and the method for producing it

ABSTRACT

The present invention relates to a recombinant severe acute respiratory syndrome coronavirus (SARS-CoV) non-structural protein (nsp) 12 with an RNA polymerase activity, its expression vector, its preparation method, and its use. According to the present invention, a soluble recombinant SARS-CoV nsp12 with an RdRp activity of initiating SARS-CoV genome synthesis can be over-expressed in the transformed host cells, and conveniently purified with high purity. An in vitro replication system important for studying SARS-CoV replication can be established with the purified recombinant SARS-CoV nsp12. SARS-CoV nsp12 produced by the present invention can also be used as a target for the development of anti-viral agents against SARS-CoV. In addition, materials inhibiting RNA-dependent RNA polymerase (RdRp) activity of nsp12 can be screened efficiently according to the present invention as the optimal conditions for the RdRp assay with SARS-CoV nsp12 were found.

RELATED APPLICATIONS

This application is a 35 U.S.C. §371 national phase application of PCT/KR2008/003333 (WO 2009/151165 A1), filed on Jun. 13, 2008, entitled “Recombinant Sars-Cov nsp12 and the Use of Thereof and the Method for Producing it,” which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a recombinant severe acute respiratory syndrome coronavirus (SARS-CoV) non-structural protein (nsp) 12 with an RNA polymerase activity, its preparation method, and its use.

Incorporated by reference herein in its entirety is the Sequence Listing entitled “sequence list.txt,” created Dec. 10, 2010, size of 21 kilobytes.

BACKGROUND ART

Since the first coronavirus infecting chickens was reported in 1937, about 15 coronaviruses belonging to the Coronaviridae family have so far been found to infect humans and animals including cattle, pigs, cats, dogs, birds, rodents, etc. Infections of animals like cattle, pigs, chickens, horses, etc. have major economical impacts on the animal husbandry industry. Particularly, porcine epidemic diarrhea, transmissible gastro-enteritis, and infectious bronchitis, caused by various animal coronaviruses, spread rapidly. Infection of infant animals is often associated with a high mortality rate. Currently, there are no effective therapeutic agents for coronavirus infection, which is economically damaging the animal husbandry industry. Regarding the coronaviruses infecting humans, human coronavirus is known to be responsible for 10-30% of all common colds in adults. SARS-CoV, emerged in the Guangdong province of China in November 2002, and spread to 26 countries worldwide including Hong Kong, Singapore, Vietnam, Canada, USA, etc. It became a worldwide health concern, with a significant impact on the economy as well. SARS is a respiratory disease caused by SARS-CoV, which is a coronavirus variant and is associated with symptoms such as fever, cough, respiratory distress, atypical pneumonia and the like. The latent period of SARS-CoV is about 2-7 days and 10-20% of patients suffer from acute respiratory distress syndrome with a death rate of 7-8%. According to World Health Organization (WHO) report in 2003, approximately 8,000 patients were found to be infected with SARS-CoV and approximately 770 of them died during the SARS outbreak in 2003. Except for several suspected SARS cases, additional infection has not been reported since April 2004. However, WHO and Center for Disease Control and Prevention (CDC) have been keeping a close watch on SARS as there are possibilities of re-emergence.

SARS-CoV belongs to the Coronavirus genus of the Coronaviridae family and has approximately 29.7 kb of positive-stranded RNA genome with cap-structure at 5′-end and poly(A) tail at 3′-end. SARS-CoV has 14 open reading frames (ORF) and 9 intergenic sequences flanked by 5′ and 3′ untranslated regions (UTRs) which are essential cis-acting elements for viral replication. A total of eight subgenomic RNAs are additionally transcribed from intergenic sequences to produce eight structural and accessory proteins. The ORF1 is composed of ORF1a and ORF1b, and the latter is translated by −1 ribosomal frameshift as a polyprotein, which is further processed by two viral proteases to generate a total of 28 viral proteins (Snijder et al., J. Mol. Biol. 331:991-1004).

It is known that RNA-dependent RNA polymerase (RdRp) plays a pivotal role in viral replication. The RdRp, probably with the help of various cellular proteins, initiates viral replication by recognizing cis-acting elements of viral RNA genome in infected cells. SARS-CoV nsp12 is encoded as a first protein in ORF1b and generated by processing of the above described polyprotein by 3C-like proteinase (nsp5). SARS-CoV nsp12 has the SDD motif that is common to coronavirus RdRps, suggesting that replication of SARS-CoV RNA genome might be mediated by nsp12 RdRp that is not present in human cells and is encoded by the viral genome.

In the absence of vaccines and therapeutic agents for various coronaviruses, development of functional RdRps is desperately needed to screen for inhibitors that can be used as antiviral agents. Moreover, antiviral drug screening in cell culture systems using a highly infectious SARS-CoV with a high mortality rate has various practical application limitations. Therefore, in vitro drug screening systems need to be developed. Indeed, tremendous efforts have been made to establish such systems using purified recombinant viral RdRps of SARS-CoV or other coronaviruses belonging to the Coronavirus genus and Torovirus genus in the Cornaviridae family. However, successful cases have not yet been reported. Even with the mouse hepatitis virus for which the coronavirus replication mechanism has been studied extensively since the 1980's, recombinant RdRp has not yet been developed and so in vitro RdRp assay systems have not been established. This is likely due to (1) low expression level of RdRp, (2) its insolubility when over-expressed, and (3) inability of purified RdRps to recognize and transcribe viral RNA templates. Recently, glutathione S-transferase-fused SARS-CoV RdRp (nsp12) was expressed and purified from E. coli. The recombinant protein was expressed mainly in insoluble form and cleaved into several fragments. Moreover, the authors did not demonstrate that the fusion RdRp protein, yet partially purified with its several cleaved forms, was able to copy RNA templates derived from the viral RNA genome (Cheng et al., Virology 335:165-176). Similarly, a purified recombinant RdRp of equine arteritis virus, which belongs to the Arterivirus genus in Nidovirale order, was not shown to be able to copy viral genome-derived RNA templates (Beerens et al., J. Virol. 81:8384-8395). In vitro replication systems for coronaviruses could be useful in studying viral RNA replication mechanisms, identifying target sites of antiviral agents by mapping of cis-acting elements, and screening for inhibitors against RdRp. Functional recombinant RdRp is the key element of the in vitro replication system. However, previous studies including the works described above have not yet established a robust in vitro replication system using a soluble, purified functional RdRp capable of utilizing the 3′-end RNA regions on both plus- and minus-strands of viral RNA, as templates.

DISCLOSURE

[Technical Problem]

The purpose of the present invention is to provide a recombinant SARS-CoV nsp12 capable of initiating SARS-CoV genome RNA synthesis.

Another purpose of the present invention is to provide an expression vector for expression of the recombinant SARS-CoV nsp12 in host cells.

One of the other purposes of the present invention is to provide a host cell transformed with the expression vector for expressing the recombinant SARS-CoV nsp12.

Yet another purpose of the present invention is to provide a method of preparing soluble recombinant SARS-CoV nsp12 efficiently.

Finally, the last purpose of the present invention is to provide the compositions and methods for screening inhibitors of SARS-CoV nsp12 RdRp.

[Technical Solution]

The present invention provides a recombinant SARS-CoV nsp12 protein in which nine amino acids are deleted from the N-terminus predicted by bioinformatics analysis, and amino acid sequences comprising several histidine residues are fused to the N-terminus.

The present invention provides an expression vector comprising a gene encoding the recombinant SARS-CoV nsp12.

The present invention provides a host cell transformed with the expression vector comprising the gene encoding the recombinant SARS-CoV nsp12.

The present invention provides a method of preparing the recombinant SARS-CoV nsp12 comprising:

-   -   culturing cells transformed with an expression vector comprising         the gene encoding SARS-CoV nsp12, in which nine amino acids are         deleted from the N-terminus and amino acid sequences comprising         several histidine residues are fused to the N-terminus;     -   inducing the expression of SARS-CoV non-structural protein         nsp12;     -   lysing cultured transformed cells; and

isolating and purifying SARS-CoV non-structural protein nsp12 from cell lysates. The present invention provides a composition for screening inhibitors against RNA-dependent RNA polymerase (RdRp), comprising SARS-CoV nsp12, MnCl₂, and NTP (nucleoside triphosphate).

The present invention provides a method of screening an inhibitor of RdRp, comprising the following steps:

-   -   a) incubating a candidate compound with the above described         composition for screening inhibitors against RNA-dependent RNA         polymerase (RdRp); and     -   b) determining whether the candidate promotes or inhibits an RNA         synthesis activity of SARS-CoV nsp12.

The present invention is illustrated in more detail below.

The present invention provides a recombinant SARS-CoV nsp12 that lacks nine amino acids at the N-terminal encoded by the 3′-end region of the ORF1a upstream of the slippery sequence required for −1 frameshift, and amino acid sequences comprising several histidine residues are fused to the N-terminus.

SARS-CoV nsp12 is expected to have amino acid sequences at the N-terminus encoded by the ORF1a because of the programmed −1 frameshift induced by an RNA secondary structure, which is formed by the sequences derived from the junction region of two ORFs, namely ORF1a and ORF1b (Plant et al., PLoS Biol. 3:e172). In the present invention, the recombinant SARS-CoV nsp12 is produced by cloning of the gene starting downstream of the slippery sequence responsible for the −1 frameshift. The resulting protein thus lacks nine N-terminal amino acids (SADASTFFK) compared with the authentically processed nsp12 protein N-terminus. In addition, the protein is expressed as a fusion protein with several histidine residues at the N-terminus.

In the present invention, SARS-CoV nsp12 is fused to several histidine residues at the N-terminus for its convenient isolation and purification. The histidine-tag was used because it can be added by a simple cloning procedure, and it is unlikely to interfere with the activity of nsp12 and binds less cellular proteins due to its small size. The number of histidine residues fused at the N-terminus is not restricted if the tagged protein can be conveniently purified using purification columns without affecting enzyme activity of SARS-CoV nsp12. For example, 6-8 histidine residues may be added to the N-terminus of nsp12. In an embodiment of the present invention, the recombinant SARS-CoV nsp12 was produced as a fusion protein with six histidine residues at the N-terminus.

The amino acid sequences comprising several histidines may further comprises several extra amino acid sequences, in addition to the histidine-tag at the N-terminus of SARS-CoV nsp12. Such amino acid sequences can originate from an expression vector used for over-expression of the recombinant SARS-CoV nsp12 and are routinely introduced as extra sequences during cloning procedures. Thus, those extra amino acid sequences can be variable depending on the types of expression vectors, and as many as 2-10 amino acids can be introduced before or after the histidine-tag, which should not interfere with the activity of recombinant SARS-CoV nsp12.

In the following examples, an expression vector for expression of recombinant SARS-CoV nsp12 was constructed and recombinant nsp12 was expressed in E. coli under optimal conditions for the production of soluble nsp12. The soluble nsp12 protein was then purified by chromatography using various purification columns. In the present invention, the recombinant nsp12 to which a histidine tag is fused, allows convenient, rapid purification of the tagged protein. Enzymatic activity of the purified nsp12 was then measured using a poly(A) RNA template, and the ability of RNA synthesis initiation from SARS-CoV genome was assessed using the 3′(+)UTR RNA, which represents the 3′-UTR of positive strand viral RNA genome, and the 3′(−)UTR RNA, which represents the 3′-end region of negative strand viral RNA complementary to the 5′-UTR of positive strand genome. The assay results showed that the recombinant SARS-CoV nsp12 in the present invention has an RdRp activity useful for the establishment of an in vitro RNA replication system.

In the present invention, the recombinant SARS-CoV nsp12 can be expressed by a vector producing a functional RdRp that is able to copy the viral RNA and can be purified conveniently.

As such a vector, the present invention provides an expression vector comprising a gene encoding a recombinant SARS-CoV nsp12 in which nine amino acids are deleted from the N-terminus and amino acid sequences comprising several histidine residues are fused to the N-terminus.

In order to prepare the above expression vector, total RNA containing SARS-CoV (Urbani strain) genomic RNA is extracted from the virus-infected cells and the gene encoding the downstream of slippery sequence, lacking nine amino acids at the N-terminus of SARS-CoV nsp12 is obtained by RT-PCR. The PCR product is cloned into a well-known expression vector using restriction enzyme digestion. Such well-known expression vectors can be chosen depending on host cell types used for expression of the recombinant nsp12. For cloning of the nsp 12-coding gene, the gene can be manipulated to encode several histidine residues at the N-terminus of nsp12, Alternatively, expression vectors designed to encode several histidine residues at the N-terminus of target proteins can be used. These expression vectors include, for example, pQE vector (Qiagen), pET vector (Novagen), pRSET vector (Invitrogen), pTrc vector (Invitrogen), etc. An embodiment of the present invention provides the expression vector pTrcSARSnsp12 depicted in the restriction enzyme map shown in FIG. 1. In the pTrcSARSnsp12, the gene encoding the nsp12 and lacking the nine amino acid residues from the authentically processed N-terminal end of SARS-CoV nsp12 was inserted into pTrcHisB, a well-known expression vector designed to produce a fusion protein with several histidine residues at the N-terminus. As shown in FIG. 1, the above expression vector pTrcSARSnsp12 has the cDNA for the specified SARS-CoV nsp12 described above that was inserted into NheI/BamHI-cleaved pTrcHisB vector. To construct pTrcSARSnsp12, total RNA from SARS-CoV-infected cells is extracted and the gene encoding the above nsp12 is obtained by RT-PCR. Then, the product digested with restriction enzymes is inserted into the pTrcHisB vector to express a recombinant fusion protein with six histidine residues at the N-terminus. The expression vector pTrcSARSnsp12 enables the SARS-CoV nsp12 to be expressed in the fusion protein with six histidine residues at the N-terminus. SARS-CoV fused to histidine is conveniently purified by using a purification column capturing proteins with histidine residues.

Although pTrcSARSnsp12 was described as an example of an expression vector designed to express a (His)₆-tagged nsp12 in the present invention, any type of expression vector, which is designed to express the nsp12 with histidine residues as a tag for convenient purification, can be used.

The recombinant SARS-CoV nsp12 in the present invention includes the recombinant nsp12 expressed by pTrcSARSnsp12 shown in FIG. 1. The recombinant nsp12 expressed by the pTrcSARSnsp12 may have the amino acid sequence of SEQ ID NO: 1. The amino acid sequence of SEQ ID NO: 1 is obtained by deleting nine amino acids from the N-terminus of the amino acid sequence of SEQ ID NO: 2 that is an authentic full amino acid sequence, and fusing six histidine (indicated by bold letters) and extra amino acids (MGGSHHHHHHGMA) that are additionally inserted into the N-terminus during cloning procedures. The recombinant SARS-CoV nsp12 comprising SEQ ID NO: 1 sequence is only an example of the present invention, and the N-terminus amino acid sequences including histidine residues can be variable in terms of the sequence length of histidine residue and the compositions for the extra sequences introduced during cloning procedures.

The present invention provides a host cell transformed with an expression vector comprising the gene encoding SARS-CoV nsp12, in which nine amino acids are deleted from the N-terminus and amino acid sequences comprising several histidine residues are fused to the N-terminus. Any type of host cell, which can be cultivated easily and efficiently express recombinant proteins, can be used to produce the recombinant SARS-CoV nsp12. As examples of such a host cell, there are microorganisms such as bacteria and yeasts, insect cells, and animal cells. In an embodiment of the present invention, the host cell may be an Escherichia coli (E. coli) cell. In one example of the present invention, E. coli TOP10 cells were used as host cells for transformation with pTrcSARSnsp12, but host cells that can be used in the present invention are not limited to this particular E. coli strain. The present invention also provides a method of preparing a recombinant SARS-CoV nsp12 comprising: culturing cells transformed with an expression vector comprising the gene encoding SARS-CoV nsp12, in which nine amino acids are deleted from the N-terminus and amino acid sequences comprising several histidine residues are fused to the N-terminus; inducing the expression of SARS-CoV non-structural protein nsp12; lysing cultured transformed cells; and isolating and purifying SARS-CoV non-structural protein nsp12 from cell lysates.

The host cells transformed with the expression vector of the present invention are cultivated at the growth conditions and media optimal for each host. Induction conditions for expression of recombinant nsp12 can be variable depending on the expression modes of the expression vectors encoding the nsp12.

According to the present invention, osmotic stress or co-expression of chaperone proteins can enhances the production of soluble nsp12 during induction of nsp12 expression. Previous attempts to establish in vitro RNA replication systems have been unsuccessful for many RNA viruses due to very low expression level and low solubility of many viral RdRps when over-expressed. However, using the methods developed in the present invention, a higher amount of soluble nsp12 can be produced. As disclosed in the following experimental example, solubility of nsp12 was enhanced by the addition of bateine and/or sorbitol to trigger osmotic pressure or by co-expression of various chaperone proteins.

After induction of the nsp12 protein expression, host cells are lysed and the nsp12 is purified from cell lysates using the purification column capturing proteins with histidine residues.

Any kind of column for selectively capturing the proteins with histidine residues as a tag can be used for purification of the nsp12. A few examples of such columns include Ni-Sepharose, HisPur cobalt resin, Talon resin, etc. In one embodiment of the present invention, Ni-nitrilotriacetic acid (NTA)-Sepharose column recognizing histidine residues is used. Additional purification columns can be used to improve the purity of the nsp12. Such additional columns include, for example, ion-exchange columns, gel-filtration columns etc. As ion-exchange columns, Q-Sepharose, DEAE (diethylamino ethyl)-Sepharose, CM (carboxymethyl)-Sepharose, and SP (sulphopropyl)-Sepharose columns, for example, can be used. As gel filtration columns, Sephacryl-, Sephadex-, and Superpose-columns can be used. In one embodiment of the present invention, Q-Sepharose column can be used as an additional purification column.

Inventors of the present invention determined the optimal RdRp activity assay conditions for SARS-CoV nsp12 in the course of characterization of the activity of recombinant SARS-CoV nsp12. As disclosed in the following example, manganese ion is inevitable for optimal RdRp activity of SARS-CoV nsp12. The optimal conditions for RdRp assay using the nsp12 are of significant importance not only for establishment of in vitro RNA replication systems but also for screening for anti-viral agents against SARS-CoV nsp12 RdRp. More specifically, inhibitory effects of potential anti-viral agents can be evaluated by assessing the level of RNA synthesized by the nsp12 in the presence of a candidate compound, under the optimized RdRp assay conditions. Thus, the present invention provides a composition for screening inhibitors against RdRp, comprising SARS-CoV nsp12, MnCl₂, and NTP (nucleoside triphosphate). The present invention also provides a method of screening an inhibitor of RdRp, comprising the following steps: a) incubating a candidate compound with the composition for screening an inhibitor of RdRp; and b) determining whether the candidate promotes or inhibits an RNA synthesis activity of SARS-CoV nsp12. In order to screen for inhibitors of the RdRp, the methods can assess whether a candidate compound promotes or inhibits an RNA synthesis activity of nsp12 by incubation of the above composition with a candidate compound in the optimal conditions of RdRp activity. Thus, nucleoside triphosphate (NTP) required for RNA synthesis and MnCl₂ essential for optimal RdRp activity can be comprised in the composition for screening for inhibitors against RdRp activity of SARS-CoV nsp12. In addition, RNA templates, primers, and buffers for RNA synthesis can be comprised in the composition for screening for an inhibitor of nsp12 RdRp. For convenient measurement of the level of RNA synthesis by SARS-CoV nsp12 in the presence of a candidate compound, NTPs can be labeled with markers. For example, such markers include fluorescence or luminescence materials or radioactive isotopes. In one embodiment of the present invention, NTPs can be ATP, GTP, CTP, and UTP, labeled with digoxigenin, biotin, or fluorescein, or their mixtures. In another embodiment of the present invention, NTPs can be ATP, GTP, CTP, and UTP, labeled with radioactive isotopes, or their mixtures. For example, RNA synthesis can be readily monitored by radiation measuring instruments detecting the radioactive NTPs. Thus, the present invention provides a method of screening for inhibitors of nsp12 RdRp, comprising incubating a candidate compound with the composition; and determining whether the candidate promotes or inhibits RNA synthesis activity of SARS-CoV nsp12 by measuring whether the RNA labeled with a marker nucleotide is synthesized or not.

According to the present invention, a high-throughput screening (HTS) system could be established for the screening of candidate compounds for inhibitors against SARS-CoV RdRp.

The advantages and features of the present invention and the method of revealing them will be explicit from the following examples described in detail. However, it is to be distinctly understood that the present invention is not limited thereto but may be otherwise variously embodied and practiced. It is obvious that the following examples are to complete the disclosure of the invention and to indicate the scope of the present invention to a skilled artisan completely, and the present invention will be defined only by the scope of the claims.

[Advantageous Effects]

According to the present invention, a soluble recombinant SARS-CoV nsp12 with an RdRp activity of initiating SARS-CoV genome synthesis can be over-expressed in the transformed host cells, and conveniently purified with high purity. An in vitro replication system important for studying SARS-CoV replication can be established with the purified recombinant SARS-CoV nsp12. SARS-CoV nsp12 produced by the present invention can also be used as a target for the development of anti-viral agents against SARS-CoV. In addition, materials inhibiting RdRp activity of nsp12 can be screened efficiently according to the present invention as the optimal conditions for the RdRp assay with SARS-CoV nsp12 were found.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of the structure of recombinant nsp12 expression vector pTrcSARSnsp12 constructed in the present invention.

FIG. 2 is a schematic diagram of the construction procedures of pTrcSARSnsp12 vector.

FIG. 3 shows the results of Coomassie blue staining of SDS-PAGE gel (top panel) and Western blot analysis with anti-(His)₅ antibody (bottom panel). Total cell lysate (T), soluble supernatant fraction (S), and pellet fraction (P) of cell lysates of E. coli TOP10 cells, which were transformed with pTrcSARSnsp12 and grown with various concentrations of sortitol, or TOP10 cells, which co-express GroES/GroEL chaeperones, were resolved by SDS-PAGE. The (−) symbol indicates the samples from the E. coli transformants that were not treated with sortitol and were not co-transformed with a vector expressing GroES/GroEL chaeperones.

FIG. 4 shows the results of electrophoresis of the fractions eluted by gradient concentrations of imidazole (0 to 500 mM) for the purification of SARS-CoV nsp12.

FIG. 5 shows the results of electrophoresis of the purified SARS-CoV nsp12 resolved by SDS-PAG (left panel) or Western blot analysis using anti-(His)₅ antibody (right panel).

FIG. 6 shows the result of an RdRp activity assay performed with poly(A) RNA template using the purified SARS-CoV nsp12 in the absence (−) or presence (+) of oligo(U)₁₂ primer.

FIG. 7 shows the effect of manganese ion on the purified SARS-CoV nsp12 activity.

FIG. 8 shows the effect of potassium ion on the purified SARS-CoV nsp12 activity.

FIG. 9 shows the time course of RdRp reaction by the purified SARS-CoV nsp12 activity.

FIG. 10 shows the effect of pH on the purified SARS-CoV nsp12 activity.

FIG. 11 shows the effect of temperature on the purified SARS-CoV nsp12 activity.

FIG. 12 shows the results of RdRp assay of SARS-CoV nsp12 performed with SARS-CoV genome-derived RNA template and poly(A) tail function analysis.

FIG. 13 shows the results of RdRp assays with (+)3′UTR151ΔA and (−)3′UTR121 RNA templates, which are derived from SARS-CoV genome.

MODE FOR INVENTION EXAMPLES Example 1

Construction of the Recombinant SARS-CoV nsp12 Expression Vector

In order to obtain SARS-CoV nsp12 with RdRp activity, SARS-CoV RNA was isolated from SARS-CoV-infected cells (FDA) by the following method, thereby preparing a cDNA fragment encoding SARS-CoV nsp12. Total RNA including viral genomic RNA was extracted from SARS-CoV-infected cells using Trizol LS reagent (Invitrogen Life Technologies). Extracted RNA was precipitated with phenol/chloroform and isopropanol successively, washed once with 70% ethanol, and dissolved in RNase-free water. The RNA was reverse-transcribed using ImProm-II Reverse Transcription System (Promega) and R1 primer, thereby synthesizing cDNA. RNA and R1 primer were incubated for 5 min at 70° C. and chilled quickly on ice. The mixtures were incubated with reverse transcription buffer, 1 mM dNTP, and 20 U of ImProm-II reverse transcriptase (Promega) at 42° C. for 60 min. After reverse-transcription, enzymes were inactivated by incubation at 70° C. for 15 min.

The reverse-transcribed cDNA having the nucleic acid sequence of SEQ ID NO:3 was amplified by polymerase chain reaction (PCR) using F1 and R1 primers with the following conditions: total 25 cycles of 94° C. for 30 sec, 55° C. for 30 sec, and 72° C. for 3 min. The PCR product was digested with XbaI and BamHI and ligated into the pTrcHisB (Invitrogen) vector digested with NheI and BamHI to construct the plasmid pTrcSARSnsp12 (about 7 kbp) encoding the nsp12 with a (His)₆-tag at N-terminus (FIG. 1).

(SEQ ID NO: 4) F1 primer: 5′-GCTCTAGAGTTTGCGGTGTAAGTGCAGC-3′ (SEQ ID NO: 5) R1 primer: 5′-CGGGATCCTACTGCAAGACTGTATGTGG-3′ cDNA for SARS-CoV nsp12 inserted in the recombinant expression vector pTrcSARSnsp12 has been cloned to comprise downstream of the slippery sequence where the programmed −1 ribosomal frameshift occurs. Therefore, the recombinant SARS-CoV nsp12 has the amino acid sequence of SEQ ID NO: 1 which is obtained by deleting nine amino acids (SADASTFFK) from the N-terminus of the amino acid sequence of SEQ ID NO: 2 that is an authentic full amino acid sequence, and fusing the amino acid sequence comprising six histidine (MGGSHHHHHHGMA)

Example 2

Improvement of the Solubility of Recombinant SARS-CoV nsp12 Protein

Like the other coronavirus RdRps, SARS-CoV nsp12 is mainly expressed as inclusion bodies, instead of soluble forms, causing difficulty in purification. In the present invention, the solubility of recombinant nsp12 was enhanced by induction of osmotic stress with betaine or sorbitol or by co-expression of chaperone proteins assisting the folding of protein tertiary structures. SARS-CoV nsp12 was over-expressed by cultivating E. coli cells transformed with pTrcSARSnsp12, in LB broth containing betaine or sorbitol or by cultivating E. coli cells co-transformed with pTrcSARSnsp12 and a vector expressing a chaperone protein(s). DnaK, DnaJ, GrpE, GroES, GroEL, TF etc. could be used as chaperone proteins. The solubility of recombinant SARS-CoV nsp12 expressed under different conditions described above was assessed by Coomassie blue staining of the SDS-polyacrylamide gel and Western blot analysis. After sonication of the cells over-expressing the nsp12, total cell lysate (T), supernatant fraction (S), and pellet fraction (P) were resolved by SDS-polyacrylamide gel electrophoresis, and subjected to Coomassie blue staining and Western blot analysis. As shown in FIG. 3, addition of sorbitol or co-transformation with GroES/GroEL significantly enhanced the level of soluble form nsp12 protein (FIG. 3.)

Example 3

Purification of Recombinant SARS-CoV nsp12 Protein from E. Coli

SARS-CoV nsp12 protein was expressed in E. coli TOP10 cells (Invitrogen) transformed with pTrcSARSnsp12. The cells were harvested by centrifugation (at 8,000 rpm) and washed twice with phosphate-buffered saline (pH 7.4, PBS) and resuspended in buffer A (50 mM Na-phosphate [pH 8.0], 300 mM NaCl, 10 mM imidazole, 10 mM β-mercaptoethanol, 10% glycerol, 1% Nonidet P-40) and frozen at −80° C. After thawing, the cells were lysed through ultrasonic lysis in an ice bath. The cell lysate was centrifuged for 20 min at 15,000 rpm to obtain clear lysate. The SARS-CoV nsp12 protein was purified by metal affinity chromatography using Ni-nitrilotriacetic acid (NTA)-Sepharose (Qiagen) resin equilibrated with the above buffer A. Unbound proteins were removed by washing the resin with 20 column volume of buffer A. The bound SARS-CoV nsp12 proteins were step-eluted with 10 ml of buffer A containing 20 to 500 mM imidazole. SARS-CoV nsp12-containing fractions were collected and purified proteins were resolved on SDS-PAGE gels (FIG. 4).

For further purification, collected fractions were transferred to a Q-Sepharose column (Amersham Biosciences) equilibrated with buffer B (50 mM Tris-HCl [pH 7.5], 50 mM NaCl, 1 mM DTT, 10% glycerol). Unbound proteins were removed by washing the resin with 20-column volumes of buffer B. Adsorbed proteins were eluted with buffer B having a linear gradient of NaCl from 0.1 to 1 M. SARS-CoV nsp12-containing fractions were collected, then proteins were resolved on SDS-PAGE gels and analyzed by Western blot (FIG. 5).

Example 4

Enzymatic Assays and Analysis of Products

Enzymatic activity of nsp12 purified as in Example 3 was measured by analysis of radioisotope-labeled RNA products subjected to denaturing polyacrylamide gel electrophoresis. Initial activity assay was performed in an RdRp assay buffer, described previously (Oh et al., J. Virol. 73:7694-7702), with 2 mM MnCl₂. After confirmation of the enzymatic activity of nsp12, optimal reaction conditions were established as in Example 5 and used for further characterization of the nsp12 in Example 4. RdRp assays were performed with 500 ng of purified SARS-CoV RdRp in a total volume of 25 μl containing 50 mM Tris-HCl (pH 7.5), 50 mM NaCl, 2 mM MnCl₂, 1 mM DTT, 10% glycerol, 20 units of RNase inhibitor (Promega), 1 μg of poly(A) template, 10 pmole of oligo(U)₂₀ and 3 μCi of [α-³²P] UTP (3,000 Ci/mmol; Amersham Biosciences). The reaction mixture was incubated for 2 hr at 32° C. After reactions, 35 μl of distilled water containing 20 μg of glycogen (Roche) and 60 μl of an acidic phenol emulsion (phenol, chloroform, 10% SDS, 0.5 M EDTA [1:1:0.2:0.4]) were added to the reaction mixture to stop the reactions. After centrifugation, RNAs in supernatant were precipitated with 5 M ammonium acetate-isopropanol (1:5), washed with 70% cold ethanol and air-dried. RdRp reaction products were resuspended in a denaturing loading buffer (95% formamide, 10 mM EDTA, 0.025% each of xylene cyanol and bromopheonol blue). After heat denaturation and quick chilling on ice, the RNA products were resolved on 8 M urea-5% polyacrylamide gels. The dried gels were exposed to X-ray film (BioMax. XAR, Kodak) for autoradiography (Oh et al., J. Virol. 73:7694-7702).

The results demonstrate that the purified SARS-CoV nsp12 incorporated radioactivity to the RNA products only when the primer is present, indicating the primer-dependent RNA synthesis (FIG. 6).

Example 5

Optimization of Enzymatic Assay for SARS-CoV nsp12

Optimal RdRp activity assay conditions for the nsp12 were determined using the poly(A) RNA template and biotin-labeled oligo(U)₁₂ primer as described in Example 4. Optimal manganese ion (Mn²⁺) concentration, potassium ion (K⁺) concentration, reaction time, temperature, and pH for RdRp activity of nsp12 were determined. RdRp assays were performed with 500 ng of purified SARS-CoV RdRp in a total volume of 50 μl containing 50 mM Tris-HCl (pH 7.5), 50 mM NaCl, 2 mM MnCl₂, 1 mM DTT, 10% glycerol, 20 units of RNase inhibitor (Promega), 1 μg of poly(A) template, 10 pmole of biotin-labeled oligo(U)₁₂ and 3 μCi of [α-³²P] UTP (3,000 Ci/mmol; Amersham Biosciences). The reaction mixture was incubated for 2 hr at 32° C. on a streptavidin-coated Flashplate (PerkinElmer Life Sciences). After reaction, the plate was washed twice with PBS and cpm (count per minute) of isotopes incorporated into synthesized RNA was measured by Top Counter (PerkinElmer Life Sciences). The results shown in FIGS. 7-11 demonstrate that the optimal RdRp activity was observed with 2 mM MnCl₂ and at a temperature of 30-34° C. and pH of 7-8.

Example 6

SARS-CoV Genome-Derived SARS-CoV 3′(+)UTR RNA Template Preparation

The 3′(+)UTR RNA, which was used as a SARS-CoV genome-derived RNA template for RdRp activity test in Example 7, was prepared by in vitro transcription using T7 RNA polymerase. SARS-CoV genomic RNA obtained as described in Example 1 was reverse-transcribed using the R3 primer. The resulting cDNA was used to obtain the 3′(+)UTR DNA template flanking 17 adenosines by PCR (total 25 cycles of 94° C. for 30 sec, 55° C. for 30 sec, 72° C. for 1 min) with the F3 and R3 primers. Similarly, 3′(+)UTR DNA template lacking the adenosine tail was also amplified by PCR using the F3 and R4 primers. These PCR products were inserted into pCR2.1-TOPO vector (Invitrogen) to construct pTOPO 3′(+)UTR-1 and pTOPO 3′(+)UTR-2 clones. After removal of T7 RNA polymerase promoter region from the pTOPO 3′(+)UTR-1 and pTOPO 3′(+)UTR-2 by restriction enzyme XhoI and BglII treatment, PCR was performed with a set of primers described above. RNA templates for RdRp assay were generated using BsaI-treated PCR DNA templates by in vitro transcription using the T7 Megascript kit (Promega). After in vitro transcription, DNA templates were removed by treatment with DNase (Ambion) at 37° C. for 30 min. RNA transcripts were extracted by phenol/chloroform (Sigma) and precipitated using isopropyl alcohol. The concentration of purified RNA templates was estimated by measuring the absorbance at 260 nm. In vitro transcribed RNAs from the PCR product obtained from pTOPO 3′(+)UTR-1 as described above has 339 nucleotides of SARS-CoV genome 3′-UTR with 17 adenosines and thus named 3′(+)UTR339+17A. In vitro transcribed RNAs from the PCR products obtained from pTOPO 3′(+)UTR-2 has only 339 nucleotides of SARS-CoV genome 3′-UTR lacking the poly(A) tail and thus named 3′(+)UTR339ΔA.

F3 primer: (SEQ ID NO: 6) 5′-TAATACGACTCACTATAGG ACACTCATGATGACCACAC-3′ (SEQ ID NO: 7) R3 primer: 5′-GGTCTCTTTTTTTTTTTTTTTTTT-3′ (SEQ ID NO: 8) R4 primer: 5′-GTCCATTCTCCTAAGAAGCTA-3′

In the above primers, the underlined nucleotides are a T7 promoter sequence for transcription, and complimentary sequences to SARS-CoV genome are indicated in bold, capital letters.

Example 7

Analysis of RdRp Activity SARS-CoV nsp12 on SARS-CoV Genome-derived RNA Templates and Function of Poly(A)-tail on RNA Synthesis

RdRp activity assay for the nsp12 purified as described in Example 3 and analysis of poly(A)-tail function were performed by measuring the amount of incorporated radioactive ribonucleotides in RNA products. More specifically, the RdRp assays were performed with 500 ng of purified SARS-CoV nsp12 in a total volume of 25 μl containing 50 mM Tris-HCl (pH 7.5), 50 mM NaCl, 2 mM MnCl₂, 1 mM DTT, 10% glycerol, 20 units of RNase inhibitor (Promega), cold ribonucleotide mixture (0.5 mM each ATP, CTP, and GTP, and 5 μM UTP), and 10 μCi of [α-³²P] UTP (3,000 Ci/mmol; Amersham Biosciences). The reaction mixture was incubated with 500 ng of RNA templates, which were prepared as described in Example 6, for 2 hrs at 32° C. After reaction, the RNA products were extracted and analyzed by electrophoresis on 8 M urea-5% polyacrylamide gels as described in Example 4. The results revealed that SARS-CoV nsp12 could copy the 3′(+)UTR339ΔA and 3′(+)UTR339+17A RNA templates, which represent parts of SARS-CoV genome, as demonstrated by incorporation of radioactive ribonucleotides into RNA products (FIG. 12). This result also indicated that both the 3′(+)UTR339ΔA RNA lacking poly(A) tail and the 3′(+)UTR339+17A RNA templates, regardless of the presence or absence of poly(A) tail, can be used by the nsp12 for de novo initiation of RNA synthesis.

Example 8

Preparation of SARS-CoV 3′(+)UTR151ΔA and 3′(−)UTR121 RNA Templates

SARS-CoV genome-derived 3′(+)UTR151ΔA and 3′(−)UTR121 RNA, which were used as RNA templates for RdRp activity test in Example 9 below, were prepared by in vitro transcription using T7 RNA polymerase as described in Example 6. The 3′NUTR151ΔA RNA consisting of 151 nucleotides of the 3′-end of SARS-CoV genome, with no poly(A) tail, was in vitro transcribed using the DNA template amplified by PCR with F4 primer and R4 primer. The DNA template for the synthesis of 3′(−)UTR121 RNA was reverse-transcribed using the R5 primer and amplified by PCR with the F5 and R5 primers. The PCR product was used as a template for the synthesis of 121 nucleotides negative strand RNA complimentary to the 5′(+)UTR of SARS-CoV genome by in vitro transcription.

F4 primer: (SEQ ID NO: 9) 5′-TAATACGACTCACTATAGG ACCACATTTTCATCGAGGCC-3′ (SEQ ID NO: 10) F5 primer: 5′-ATATTAGGTTTTTACCTAC-3′ R5 primer: (SEQ ID NO: 11) 5′-TAATACGACTCACTATAGG TAGGTGCACTAGGCATGC-3′

In the above primer, the underlined nucleotides are a T7 promoter sequence for transcription, and complimentary sequences to SARS-CoV genome are indicated in bold, capital letters.

Example 9

Analysis of the RNA Synthesis Ability of SARS-CoV nsp12 to Use SARS-CoV Genome-Derived 3′(+)UTR151ΔA and 3′(−)UTR121 RNAs as Templates

RdRp assays were performed with SARS-CoV 3′(+)UTR151ΔA and 3′(−)UTR121 RNA templates prepared as in Example 8 by the methods described in Example 7, and the RdRp products were subjected to denaturing polyacrylamide gel electrophoresis. The results showed that the nsp12 can use both 3′(+)UTR151ΔA and 3′(−)UTR121 RNAs as templates, as demonstrated by incorporation of radioactive substrates into the RNA products. In vitro synthesis of positive strand RNAs by the nsp12 was more efficient than that of negative strand RNA (FIG. 13). This result is consistent with the notion that a greater amount of positive strand RNAs are synthesized than minus strand RNA during viral replication in the cells infected with positive strand RNA viruses. Furthermore, we were able to confirm that the nsp12 requires manganese ion for RdRp activity. 

The invention claimed is:
 1. A recombinant severe acute respiratory syndrome coronavirus (SARS-CoV) nsp12 having the amino acid sequence of SEQ ID NO:1, in which nine amino acids are deleted from the N-terminus and amino acid sequences comprising several histidine residues are fused to the N-terminus.
 2. The recombinant SARS-CoV nsp12 according to claim 1 that is expressed by the expression vector pTrcSARSnsp12.
 3. A composition for screening inhibitors against RNA-dependent RNA polymerase (RdRp), comprising the recombinant SARS-CoV nsp12 of claim 1, MnCl₂, and NTP (nucleoside triphosphate).
 4. The composition according to claim 3, wherein the recombinant SARS-CoV nsp12 is expressed by the expression vector pTrcSARSnsp
 12. 5. The composition according to claim 3, wherein the NTP includes ATP, GTP, CTP or UTP, which is labeled with a marker probe.
 6. A method of screening for inhibitors of RdRp, comprising the following steps: a) incubating a candidate compound with the composition according to claim 3; and b) determining whether the candidate compound promotes or inhibits an RNA synthesis activity of SARS-CoV nsp12.
 7. The method of claim 6, wherein the step b) comprises measuring whether an RNA labeled with a marker nucleotide is synthesized or not. 